The present invention generally relates to transmission level setting circuits and modem units, and more particularly to a transmission level setting circuit which sets a carrier transmission level when making a data communication on a switched line or a radio line of a portable telephone, and to a modem unit using such a transmission level setting circuit.
Recently, it has become possible to make a data communication using a switched line or a radio line of a portable telephone by connecting a modem unit to the portable telephone or the switched line and controlling the modem unit from a personal computer or the like. In this case, because of the structural differences between the switched line and the radio line, the methods of setting the carrier transmission level from the modem unit are different between the case where the switched line is used and the case where the radio line is used. For this reason, in the general modem unit, the setting of the carrier transmission level is changed depending on the line used.
When making the data communication via the switched line, signal attenuation occurs depending on the distance from the modem unit to an exchange. Hence, in order to set the carrier transmission level of the modem unit to an optimum value, it is necessary to take into consideration the attenuation value and to increase and set the carrier transmission level depending on the attenuation value.
In a conventional modem unit that is used to make the data communication via the switched line, a standard setting of the carrier transmission level when the modem unit is shipped from a factory is -15 dBm, for example. For this reason, when this modem unit is connected to the personal computer or the like, it is necessary to increase the carrier transmission level at a transmitter part within the modem unit depending on the signal attenuation value that is dependent on the distance from the modem unit to the exchange. Such a change in the carrier transmission level is made manually from the personal computer or the like to which the modem unit is connected, by a qualified person such as the engineer in charge of the work.
On the other hand, when making the data communication via the radio line of the portable telephone, there is no need to take into consideration the signal attenuation value because the modem unit and the portable telephone are connected within a short distance. Hence, the carrier transmission level of the modem unit can be set to the optimum value by setting the carrier transmission level to a maximum value within a range of the input level specified by the portable telephone.
A conventional modem unit that is used to make the data communication via the radio line of the portable telephone is connected to the portable telephone via an attenuator which reduces the standard carrier transmission level that is set at the time of the shipping from the factory to -51 dBm, for example, and a resistor having a transmission impedance of 600 .OMEGA.. Accordingly, when making the data communication via the radio line of the portable telephone, it is necessary to change the carrier transmission level manually from the personal computer or the like to which the modem unit is connected, by a qualified person such as the engineer in charge of the work.
According to the conventional modem units, it was necessary to manually change the carrier transmission level depending on the kind of line that is used for the data communication, and there was a problem in that it required a troublesome operation of manually changing the carrier transmission level. In addition, a person who may perform such a troublesome operation was limited to a qualified person such as the engineer in change of the work, thereby leading to another problem in that the general users were prevented from freely making the data communication using the portable telephone.
On the other hand, it is conceivable to connect between the modem unit that is connected to a the switched line and the portable telephone an adapter having a circuit which attenuates a signal in a signal path from the modem unit to the portable telephone by a fixed attenuation value of 36 dB, for example. Even in this case, however, problems similar to those of the conventional modem unit described above occur because the carrier transmission level from the adapter to the portable telephone will change if the carrier transmission level from the modem unit changes.